Process for producing pig iron

ABSTRACT

A process for producing pig iron is described involving the use of a reduction shaft furnace 1 and a melt-down gasifier 2. The sponge iron produced from iron ore in the reduction shaft furnace is fed into the melt-down gasifier and converted into a pig iron melt. The gas produced in the melt-down gasifier is passed as reducing gas into the reduction shaft furnace both via a cyclone separator 11 and a line 12 and directly via a line 13. The top gas leaving the reduction shaft furnace, after passing through a scrubber 4, is largely returned via a CO 2  scrubber 6, in which CO 2  and H 2  O are removed from the gas, in such a way that it can be used for forming the reducing gas in the melt-down gasifier and also as a cooling gas for the reducing gas produced in said gasifier.

The invention relates to a process for producing pig iron, in which theiron ore is reduced in a reduction shaft furnace and the sponge ironobtained as a result thereof is subsequently melted in a melt-downgasifier, a carbon carrier and a reducing gas being introduced into saidgasifier and the gas produced in the latter is passed as reducing gasinto the reduction shaft furnace.

German Patent 28 43 303 discloses a process for producing molten pigiron and reducing gas in a melt-down gasifier, in which sponge ironparticles are melted down to molten pig iron or steel raw material andthe heat necessary for melting and the reducing gas are produced fromcoal supplied and oxygen-containing gas blown in above the melt. In thisprocess a relatively large amount of surplus gas (export gas) isobtained and the energy consumption, i.e. the consumption of coal andoxygen is very high. If the surplus gas cannot be economically used, thepig iron production costs are very high. The chemical energy in thesurplus gas is more than 50% of the energy supplied with the coal, andthe utilization of the reducing gas produced in the melt-down gasifieris max. 44%.

It is known from U.S. Pat. No. 4,225,340 to substantially free the topgas of a reduction shaft furnace from carbon dioxide and use the thustreated top gas for producing fresh reducing gas. This is supplied tothe starting gas of a fossile fuel-processing gasifier, a gas reactorconnected downstream of the gasifier, and also the starting gas for thegas reactor both directly and via a gas heater. The starting gas of thegas reactor with the admixed treated top gas is passed in the form ofreducing gas to the reduction shaft furnace. However, this known processis only used for producing sponge iron without the use of a melt-downgasifier.

A similar sponge iron production process is known from U.S. Pat. No.4,260,412. In the latter, once again top gas of a reaction shaft furnacetreated by carbon dioxide removal is used for producing fresh reducinggas. The gas producing means is a fluidized bed gasifier, to which issupplied the treated blast furnace gas, coal and lime, as well as oxygenand optionally steam.

The starting gas of this gasifier is admixed with the treated top gasboth directly and via a gas heater, before it is passed into thereduction shaft furnace. Thus, this process also functions without amelt-down gasifier for the production of pig iron.

The problem of the present invention is to provide a process for theaforementioned type in which the consumption of coal and oxygen isconsiderably reduced.

According to the invention this problem is solved in that the top gas ofthe reduction shaft furnace is at least partly freed from oxidizedcomponents CO₂ and H₂ O and the thus treated top gas is fed into themelt-down gasifier. According to an advantageous development of thisprocess, the treated top gas is added to the gas produced in themelt-down gasifier and passed into the reduction shaft furnace, as acooling gas. Preferably a total of 50 to 95% and more specifically 85%of the top gas is to be returned for reducing gas production. Inaddition, only that amount of surplus gas (export gas) is to be producedto enable driving the compressor of the O₂ plant and/or to produce steamfor CO₂ scrubbing.

The invention is described in greater detail hereinafter relative to anembodiment shown in the drawing, which diagrammatically represents aplant for producing pig iron.

The plant contains a reduction shaft furnace 1 constructed in per seknown manner, to which is supplied from above via a not shown inlet ironore and optionally fluxing agents. Reducing gas is supplied to themelt-down gasifier 2 from below to the reduction shaft furnace 1. Themelt-down gasifier 2 once again receives the sponge iron produced by thereduction of iron ore and produces therefrom a pig iron melt and forthis purpose coal and oxygen are supplied to gasifier 2. The coal can beconstituted by low-grade coke or high temperature carbonized coke.

The top gas of the reduction shaft furnace is brought via a line 3 intoa scrubber 4 and is then mostly supplied, i.e. approximately 50 to 95%and preferably 85%, to a CO₂ steam scrubber 6 via a compressor 5. Thegas leaving the latter and which is free from CO₂ and H₂ O initiallypasses into a line 7 and from there into three branch lines 8, 9, 10.The treated top gas of the reduction shaft furnace removed via branchline 8 is used as a cooling gas for the gas leaving the melt-downgasifier 2. It cools this gas from approximately 1000° C. toapproximately 850° C. In branch line 9, the treated top gas is used as aconveying gas for the dust separated in a cyclone separator 11 on thegas produced in the melt-down gasifier 2 and returned to the latter. Thereducing gas is firstly passed via cyclone separator 11 and a line 12and secondly directly via a line 13 from melt-down gasifier 2 to thereduction shaft furnace 1. The remainder of the treated gas is suppliedvia branch line 10 to the O₂ tuyere level of melt-down gasifier 2.

The top gas leaving scrubber 4 is only to a limited extent removed fromthe aforementioned circuit via line 14 as waste gas. This portion ispreferably kept as low as is required for removing the nitrogen suppliedto the process. Particularly when using top quality coal, e.g. hightemperature carbonized coke or anthracite, sufficient top gas can bereturned by CO₂ scrubber 6 to ensure that substantially no surplus gasis discharged. Then only that surplus gas quantity is discharged as isrequired for removing the nitrogen. The consumption of coal and oxygencan be roughly halved, compared with the conventional process withoutany top gas return. When using low quality coal, i.e. coal with a highcontent of ash and volatile constituents, it is advantageous to heat thetreated, returned top gas with a plasma burner, in order to obtainadvantageous use values.

The purified top gas leaving the CO₂ scrubber 6 and consisting of thecomponents H₂, CO and N₂, can be preheated in a heat exchanger prior tointroduction into the melt-down gasifier 2. Preheating takes place to atemperature in the range 300° to 600° C. and preferably 400° C.Preheating to higher temperatures is not advisable, because it wouldlead to decomposition of CO. The final heating then takes places inmelt-down gasifier 2.

If a fluidized bed is formed in the melt-down gasifier it is alsoadvantageous to introduce the treated top gas into the gasifier atroughly half the height of the fluidized bed.

It is also advantageous to produce sufficient surplus gas to enable theconventional compressor of the O₂ plant to be operated and/or to heatsteam for CO₂ scrubbing.

I claim:
 1. In a process for producing pig iron in which iron ore isreduced in a reduction shaft furnace issuing a top gas containing CO₂and a sponge iron obtained as a result thereof is subsequently passedthrough a fluidized bed formed within a melt-down gasifier above an O₂tuyere, a gas and a carbon carrier being introduced into said melt-downgasifier, and a reducing gas produced in said melt-down gasifier beingintroduced as the reducing gas into said reduction shaft furnace; theimprovement comprising the steps of compressing and at least partiallyscrubbing the CO₂ from the top gas issuing from said reduction shaftfurnace, and then introducing at least a portion of this scrubbed andcompressed top gas into the melt-down gasifier at about half the heightof said fluidized bed.
 2. The process of claim 1 wherein the scrubbedand compressed top gas introduced into the melt-down gasifier at abouthalf the height of said fluidized bed is preheated to a temperature ofbetween about 300° C. to about 600° C. prior to said introduction. 3.The process of claim 1 wherein the temperature of the gas uponpreheating is about 400° C.
 4. The process of claim 1 wherein anadditional portion of the scrubbed and compressed top gas is introducedinto the melt-down gasifier at the level of the O₂ tuyer.
 5. The processof claim 4 wherein a second additional portion of the scrubbed andcompressed top gas is introduced as a cooling gas into the stream ofreducing gas passing into the reduction shaft furnace.
 6. The process ofclaim 5 wherein the amount of the second additional portion of thescrubbed and compressed top gas is such as will cool said stream ofreducing gas passing into the reduction shaft furnace to a temperatureof about 850°